Transmissometers and scattering meters need to measure light at very low levels with reliability and reproducibility. Parasitic light, produced by scattering off optical surfaces or the dust on them, must be kept to a minimum and must not change with time in order that its effect may be subtracted accurately from the received signal. Unfortunately, the collecting optics for such apparatus are complex and generally involve many optical surfaces which are difficult to clean and align. This results in a large parasitic signal.
Standard optics, because of their rigid geometric disposition requirements, take up a lot of space which forces the overall device to be larger. This is merely annoying in an instrument designed for the laboratory. However, for an instrument designed to perform measurements in the field, too large a size may completely preclude or at best severely curtail its use.
In order to improve the situation, apparatus has previously been designed wherein the received beam is separated into several beams by a set of beam-splitters arranged in series. At the focus of each beam an optical mask is inserted. The optical mask for the direct transmission channel has a hole of small diameter at its center. The various scattered channels have rings cut out of them. The outer and inner diameters of the rings define the acceptance angles for the scattered light. The angles .theta. are related to the diameters D by the following formula: ##EQU1## where F is the focal length of the primary lens of the receiver. The light transmitted by each mask is then collected onto the detectors by short focal length lenses situated behind each mask. The lenses are disposed in such a way as to image the total entrance plane of the primary lens on the detectors.
In another prior art method, a special detector assembly is fabricated. This assembly consists of a set of ring detectors and a central circular detector. In this case, the angles of acceptance are related to the various inner and outer diameters of the detectors by the formula given above. The primary focusing lens and the face of the detector assembly are the only optical surfaces present in this method.
It has become apparent that the disadvantage of the first-mentioned prior art method is the large number of optical elements that have to be aligned precisely and then maintained in position rigidly even when the transmissometer-nephelometer is moved or bumped hard, as it will be in field use. The surfaces of these optical elements have to be cleaned carefully before any alignment and elaborate precautions have to be taken to ensure that they remain clean during and after the alignment. This is essential since the scattering of these surfaces will be seen by the instrument and will considerably reduce its sensitivity and accuracy. For these reasons, maintenance of such a system is a complex undertaking.
In the second prior art method mentioned above, the limitations are principally due to electronic problems. The optics are simple in this apparatus and involve only the primary focusing lens and the detector assembly. The first problem of an electronic nature is due to cross-talk between the various detectors. This means that some portion of the signal falling on a given detector will be seen by the others. Since the detectors of scattered light receive, in general, signals which are much smaller than the transmitted signal in the central detector of the array, they will be swamped by the induced cross-talk and their readings will be inaccurate. The second problem of an electronic nature is due to the noise inherent in the detectors. In forward-angle scattering the signal gets much weaker as the angle is increased and one would therefore like to collect it over as large an area as possible. However, if one increases the area of the detector the electronic noise increases as the square root of the area. The capacitances of the detector also increases as the area of the detector increases and this further increases the noise of the transimpedance amplifiers connected to them. The accuracy of the device is therefore severely compromised. This complication is avoided in the first prior art apparatus mentioned above but at the expense of using separate lenses to reduce the image of each ring with various magnifications onto detectors of the same size.
It is an object of the present invention to provide optical apparatus in which the above-mentioned disadvantages are reduced or substantially obviated.